Adjustable tuning posts disposed in a microwave cavity have been used in a number of radio frequency (RF)/microwave components, such as waveguides, TEM-lines, RF filters, resonators, etc.
Generally, the adjustment of a tuning post within an RF cavity will change the electrical characteristics of the microwave device.
High-power filters which make use of tuning posts have been used for a number of years for space applications, satellites, for examples. Unfortunately, because of the power requirements as well as the wide range of operating temperatures common to space applications, filters with adjustable tuning posts have been found to cause a number of problems.
For example, even if filter components are fixed, the metals used in filters, such as aluminum, expand and contract with the large temperature changes common in space applications, thereby modifying the filter behaviour and rapidly limiting its performance capability. In addition to meeting the thermal requirements of space applications, microwave components must often be designed to take into account problems associated with the effects of Multipaction, and Passive Intermodulation Interference (PIM).
The Multipactor effect is a vacuum discharge produced by an RF field between a pair of surfaces. Electron multipaction (avalanche) is by secondary electron emission from these surfaces. For multipaction breakdown to occur, the pressure must be sufficiently low so that the mean free path is longer than electrode separation distance. Thus, electrons can readily travel between the electrodes without undergoing collisions with gas molecules. When these electrons collide with the electrodes, they release secondary electrons provided that the primary electron possesses sufficient energy and that the electrode surfaces have a secondary emission coefficient greater than one. If this occurs as the electric field passes through zero, the reversed electric field will accelerate the electrons back across the gap. If the transit time of the electrons across the gap is one half the cycle of the RF field, the secondary electrons formed by the initial electrons become primary electrons for the next half cycle to form another group of secondary electrons. In this way, large electron densities rapidly build up in the gap and breakdown results.
Another problem encountered in space applications is the risk of interference due to PIM, especially for multichannel communication systems for which the RF output power level has significantly increased over the years.
It is well-known that harmonics and intermodulation (IM) products are generated when two or more signals are applied to a nonlinear circuit element. In a practical communication system, the harmonics and intermodulation products generated by the high-power amplifiers are effectively filtered out using output transmit filters. For mobile satellite applications, a very high transponder gain is required and high-rejection output filters providing, for example, some 100 dB suppression in the receive band are needed.
Passive components and materials used in communication satellites can exhibit nonlinear voltage/current characteristics and can generate harmonics and intermodulation products. Since these spurious signals are generated by passive components, the term Passive Intermodulation (PIM) is attributed to such spurious signals. Although these signals are produced at very low levels, the PIM signals falling into the receive frequency band can cause serious interference problems if they are generated after the output high-rejection filters in the antennas or by surrounding structures on the spacecraft, and if they are picked up by the communication system.
PIM performance is very critical for mobile satellite applications. Due to the low frequency of operation (UHF, L-Band or S-Band), waveguide technology leads to unacceptably large and heavy components, and coaxial technology must be used instead. However, very high current densities, which enhance the risk of PIM generation, exist on the centre conductor of coaxial structures.
Metal-insulator-metal (MIM) junctions that are exposed to multi-carrier signals can result in nonlinear behaviour which can cause PIM. These junctions are caused by oxides forming between metallic surfaces. Rough surfaces can prevent a good metal-to-metal contact and can also create nonlinear junctions that cause PIM. Very high-pressure contacts, or else noncontacting interfaces using dielectric insulators, are mandatory to reduce the risk of PIM. This is particularly critical at the mating interfaces of coaxial high-power components, for which a good contact must be maintained over a wide operating temperature range. Such devices include, for example, coaxial quarter-wave microwave filters.